This invention relates to demodulating apparatus and, more particularly, to demodulating apparatus of the type which includes a synchronous detector for detecting an amplitude modulated signal.
One example of a synchronous detector is described in U.S. Pat. No. 3,241,078. In using a synchronous detector to demodulate an amplitude modulated signal, it is necessary to generate or derive a local carrier whose frequency is equal to the frequency of the carrier upon which the amplitude modulating signal is modulated. This local carrier is supplied to one input of the synchronous detector and the amplitude modulated signal is supplied to another input thereof. The resultant output of the synchronous detector is the information signal which had been used to modulate the carrier in producing the amplitude modulated signal.
If the synchronous detector is used in radio wave receiving apparatus, for example, in a television receiver, a radio receiver, or the like, the received high frequency signal first is converted to an intermediate frequency (IF) signal, and the synchronous detector demodulates this IF signal. For example, when used to receive a television signal, the IF signal generally includes an IF carrier of about 58.75 MHz, which is an amplitude modulated signal. This IF signal is applied directly to one input of the synchronous detector, and a carrier deriving circuit is connected to receive the IF signal and to derive a substantially unmodulated IF carrier which is applied to another input of the synchronous detector.
In apparatus of the aforedescribed type, the IF carrier deriving circuit may include a tuned circuit followed by a series-connected limiter circuit. If the tuned circuit is tuned to the IF carrier frequency of 58.75 MHz, then this carrier will be derived and supplied through the limiter circuit to the synchronous detector. The function of the limiter circuit is to remove level variations, or modulations, in the IF carrier derived by the tuned circuit so as to supply a substantially unmodulated IF carrier to the synchronous detector. For proper operation of the synchronous detector, the phase of the local carrier which is supplied thereto by the carrier deriving circuit should be coincident with the phase of the amplitude modulated signal which also is supplied to the synchronous detector. However, the limiter circuit generally imparts a phase delay to the local carrier which is derived from the amplitude modulated IF signal.
In demodulating apparatus of the type described, it heretofore has been proposed that the inherent phase shift in the limiter circuit be compensated, or cancelled, by providing an equal and opposite phase shift in the tuned circuit. That is, if the limiter circuit imparts a phase delay, then the tuned circuit should add a phase advance to the carrier derived thereby. This is achieved by establishing the center frequency of the tuned circuit to be different from the IF carrier frequency of 58.75 MHz. It is known that a tuned circuit exhibits a particular phase-frequency characteristic such that a positive or negative phase shift is imparted to the derived carrier, the degree and direction of this phase shift being dependent upon the difference between the frequency of the derived carrier and the center frequency of the tuned circuit. For example, a positive phase shift, or phase advance, is imparted to the derived carrier if the frequency of the derived carrier is less than the center frequency of the tuned circuit. Therefore, in order to compensate for the phase delay attributed to the limiter circuit, the center frequency of the tuned circuit is set to be higher than the desired IF carrier frequency of 58.75 MHz.
One problem in establishing the center frequency of the tuned circuit to be different than the derived carrier frequency is that the amplitude of the derived carrier produced by the tuned circuit may be less than the input limiting level of the limiter circuit. That is, the amplitude-frequency characteristic of the tuned circuit may appear as a bell-shaped curve having sharply sloped flanks. Thus, since the desired carrier frequency differs from the center frequency of the tuned circuit, the corresponding amplitude of the carrier derived thereby may be relatively low. If this low amplitude carrier is less than the limiting level of the limiter circuit, the level of the carrier which is supplied to the synchronous detector may be less than the desired, limited level. Furthermore, because the carrier supplied to the limiting circuit is less than the limiting level, there may be fluctuations in the amplitude of the carrier which is applied from the limiting circuit to the synchronous detector. Consequently, proper synchronous detection of the amplitude modulated signal might not be achieved. This problem is further compounded if the tuned circuit includes a trap circuit to trap the audio signals which are modulated onto another carrier and which generally are provided in the IF signal. This trap circuit tends to further increase the slope of the flanks of the amplitude-frequency characteristic of the tuned circuit.